Pyridine → (2S)-2-(((tert-butoxy)carbonyl)amino)-3-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)propanoic acid

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Bis(trifluoroacetoxy)iodobenzene (44.4 g, 103.2 mmol) was added to a solution of N-[(1,1-dimethylethoxy)carbonyl]-L-asparagine (Boc-Asn; 20.00 g, 86.1 mmol) in N,N-dimethylformamide/water (2:1; 300 mL). The solution was stirred at room temperature for 30 min and then pyridine (10 mL, 123.6 mmol) was added. The solution was stirred at room temperature for 4.5 h and then the solvent was evaporated (45° C., ˜0.5 mm Hg). The residue was dissolved in acetone/water (1:1; 400 mL) and sodium hydrogen carbonate (31.8 g, 378.5 mmol) and 1-[[(9H-fluoren-9-ylmethoxy)carbonyl]oxy]-2,5-pyrrolidinedione (Fmoc-OSu; 34.74 g, 103.0 mmol) were added. The mixture was stirred at room temperature overnight, then the acetone was evaporated and the mixture was acidified to pH 1 with 1 M HCl and extracted with ethyl acetate (200 mL, then 100 mL). The combined organic layers were washed with brine (3×50 mL), dried (Na2SO4), filtered, evaporated (45° C., ˜0.5 mm Hg), and chromatographed (50-60% ethyl acetate/hexanes) to give N-[(1,1-dimethylethoxy)carbonyl]-3-[(9H-fluoren-9-ylmethoxy)carbonyl]amino-L-alanine (32.33 g, 88%) as a white solid.

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